<chrono>: Fix hh_mm_ss subsecond formatting for floats (#1866)

* <chrono>: Fix hh_mm_ss subsecond formatting for floats

Before, the same formatting string was used for floats and integrals.
This meant that large floats were formatted using exponent notaion and
small floats were not, and it also meant there was an extra period in a
time, as the subseconds could be fractions of a subsecond (say .4
nanoseconds). Now if the subseconds are floats, we force fixed
formatting to get the right number of leading zeroes and a precision of
0 to round off fractions of subseconds.

* Floor subseconds

stl/inc/chrono

@@ -5469,9 +5469,14 @@ namespace chrono {
     void _Write_seconds(basic_ostream<_CharT, _Traits>& _Os, const hh_mm_ss<_Duration>& _Val) {
         _Os << _STD format(_STATICALLY_WIDEN(_CharT, "{:02}"), _Val.seconds().count());
         if constexpr (hh_mm_ss<_Duration>::fractional_width > 0) {
-            _Os << _STD format(_STATICALLY_WIDEN(_CharT, "{0}{1:0{2}}"),
-                _STD use_facet<numpunct<_CharT>>(_Os.getloc()).decimal_point(), _Val.subseconds().count(),
-                _Val.fractional_width);
+            _Os << _STD use_facet<numpunct<_CharT>>(_Os.getloc()).decimal_point();
+            if constexpr (treat_as_floating_point_v<typename hh_mm_ss<_Duration>::precision::rep>) {
+                _Os << _STD format(_STATICALLY_WIDEN(_CharT, "{:0{}.0f}"), _STD floor(_Val.subseconds().count()),
+                    _Val.fractional_width);
+            } else {
+                _Os << _STD format(
+                    _STATICALLY_WIDEN(_CharT, "{:0{}}"), _Val.subseconds().count(), _Val.fractional_width);
+            }
         }
     }
 

tests/std/tests/P0355R7_calendars_and_time_zones_formatting/test.cpp

@@ -593,6 +593,14 @@ void test_hh_mm_ss_formatter() {
     empty_braces_helper(hh_mm_ss{4083007ms}, STR("01:08:03.007"));
     empty_braces_helper(hh_mm_ss{65745123ms}, STR("18:15:45.123"));
     empty_braces_helper(hh_mm_ss{65745s}, STR("18:15:45"));
+    empty_braces_helper(hh_mm_ss{0.1ns}, STR("00:00:00.000000000"));
+    empty_braces_helper(hh_mm_ss{1.45ns}, STR("00:00:00.000000001"));
+    empty_braces_helper(hh_mm_ss{1.56ns}, STR("00:00:00.000000001"));
+    empty_braces_helper(hh_mm_ss{1e+8ns}, STR("00:00:00.100000000"));
+    empty_braces_helper(hh_mm_ss{999'999.9us}, STR("00:00:00.999999"));
+    empty_braces_helper(hh_mm_ss{59'999'999.9us}, STR("00:00:59.999999"));
+    empty_braces_helper(hh_mm_ss{3'599'999'999.9us}, STR("00:59:59.999999"));
+    empty_braces_helper(hh_mm_ss{86'399'999'999.9us}, STR("23:59:59.999999"));
 
     assert(format(STR("{:%H %I %M %S %r %R %T %p}"), hh_mm_ss{13h + 14min + 15351ms})
            == STR("13 01 14 15.351 01:14:15 PM 13:14 13:14:15.351 PM"));